1. Field of the Invention
The present invention relates to a radioactive iodine compound which is used as a labeling reagent for introducing a radioactive isotope element into an antibody molecule in an immunoradiometric assay, and an intermediate for preparing the radioactive iodine compound.
2. Description of the Prior Art
When a protein of interest is detected or quantitatively determined by an antigen-antibody reaction, the antibody is often labeled by a certain material. The assay systems using the labeled antibody include an immunofluorometric assay, an immunoenzymometric assay, an immunoradiometric assay, and the like. For example, the immunoradiometric assay is a method which comprises labeling an antibody with a radioactive isotope element such as .sup.125 I, and measuring the radioactivity of the antigen-antibody complex to determine the amount of the antigen. This method has widely been utilized because a trace amount of antigen can be determined.
Examples of processes for introducing .sup.125 I into an antibody molecule in the immunoradiometric assay include the chloramine T method, the enzyme method, and the like. According to these methods, for example Na.sup.125 I is oxidized, and then the resulting free .sup.125 I is introduced into the Tyr residue present in the antibody molecule. Other methods include Bolton-Hunter reagent method. However, in these methods, some .sup.125 I may be introduced into the specific binding sites of the antibody, which may reduce the activity of antibody, resulting in a reduction of sensitivity of the measurement.
Other processes for introducing .sup.125 I into an antibody molecule include those comprising introducing .sup.125 I-containing compounds directly into the antibody molecule. The .sup.125 I-containing compounds include those represented by the following formula (II) (See; P.C. Srivastava et al, J. Nucl. Med., 31(5), 906(1990)): ##STR2##
However, this type of compound is relatively unstable. Furthermore, the synthesis of this compound requires some complicated steps.
Labeling reagents that can readily introduce .sup.125 I into an antibody molecule are required for there have been increasing a demand for detecting or determining protein of interest by using the labeled antibody described above.